24v or 12v

[Steevo25]

Hi All,

I am staring to get in to my solar panel system now and since I have it, I might as well make more use of it. As it stands, I have 2 x 100w panels in parallel that run through a 20A MPPT charger and then charge 2 x 110AH batteries in parallel (so 12v at the panels and 12v at the batteries).

My MPPT charger says that at 12V, the maximum panel wattage it can take is 275W at 12v or 550W at 24v. I was thinking of adding another 2 x 100W panels just for that extra bit of power since the UK weather is not known for it's excess of sunshine.

I assume that I could just pop another 2 panels on the roof of my hangar and then run them with 2 panels in series each and then parallel the 2 series together which will give me 24v with a potential for 400W. I also assume that because I am doubleing the voltage (as well as the wattage), that the amps will remain the same so I will not have to change any cabling or fusing.

Does this sound ok to do? The battery bank will still be run at 12v as I have items connected to it that will not run at anything other than 12v. Also, would this be more efficient anyway as 24v means lower ampage and less voltage drop. My panels are roughly about 10feet from the charge controller and then my batteries about 3feet from the charge controller (cable length). The panels are currently connected to the charge controller with 6mm cable and a 30amp fuse and then go from the charge controller to the batteries with 6mm cable and a 30amp fuse.

Many thanks in advance.

[Wy_White_Wolf]

When the charge controller states 12 or 24 V it is talking about system voltage. You will have to increase the battery bank to 24V if you wish to run 400 watts array and use the extra charging power.

WWW

[Steevo25]

I wasn't aware of that but as I say it was only an idea. There is also an FAQ section at the back of the manual that says 'Can 24v panels be used to charge 12v batteries'. The answer is 'Yes', but it doesn't say what you have to do to be able to do it. All I have in the menu of the MPPT controller is a setting that says 'System Voltage' and that can be set to either 12v or 24v. But in the specification section it says maximum PV input for controller 275W at 12v / 550W at 24v. But it also says 12v/24v auto detect.

[Logan005]

2 X 12 volt batteries run in parallel is not good, change to series and 24 volts battery bank. now also wire your panels in series. should give you 30 to 34+ volts into the MPPT CC and 29 volts into the battery bank. no need to add panels, but you will need a 24 volt to 12 volt step down converter and a 24 volt inverter. This set up is much more efficient than a 12 volt system. 12 volts is for toys and automobiles. You already have an MPPT controller, solar input voltage should be close to the CC's max. to reduce cable length losses. I have 4X 250 panels producing 150+ volts into my charge controller charging my 24volt battery bank. If you are already using an MPPT CC, forget the panel voltage and let the CC handle the difference.

[Steevo25]

That would probably be ideal and if I was just starting then I would probably plan to go that route. Unfortunately, everything I have runs on 12v including a 12v inverter so it would mean replacing all of them. A step down converter could be used for some of it but the step down converter itself would use some energy to run. The main purpose of the system is to run an alarm system 24/7 so keeping the energy used as low as possible for overnight use is the ideal. Presently, the whole of the alarm system (including a GSM camera I have just added) only uses about 110ma peak. In it's current form, the system would run for days and days (possibly weeks) without any charge if it wasn't used for anything else.

When I first bought it, I did explain to a professional solar company what I was trying to achieve and the specification they gave me for running the alarm was 1 x 105AH battery, 1 x 75w 12v panel and a PWM charge controller. This was deemed more than sufficient to purely run the alarm system. But the cost of doing what I actually purchased was not all that much more in the grand scheme of things so I ended up with 2 x 100w panels, an MPPT charge controller and 2 x 110AH batteries. It came as a complete kit including wiring etc. All I had to purchase in addition was some circuit breakers. Then I got a fair deal on a 1000w 12v to 230v inverter which will only be used very occassionly to power some lights when I don't want the noise of the generator running.

My idea now is to make a bit more use of it as even when I leave it a few days, the batteries are still fully charged so it isn't under any strain at the moment. But at present the UK estimate for peak solar is about 2 hours per day where I am so the panels are only really running efficiently for approx 2 hours everyday. So although it's quite a big system for the alarm, I will not be getting big hits from the solar. This is why I thought more panels may be the way to go just to capture more sunshine for the brief period it is actually there.

[Logan005]

Sounds like you have it all figured out then.

[Steevo25]

I never have it figured out. I have got this far with trial and error so far. I am a solar panel virgin, it's only the case that I know a fair bit about electronics that I have got as far as I have with the solar stuff.

[Logan005]

you will learn the hard way if nothing else. batteries charged in parallel will fail within a year maybe even less. your 12 volt inverter is likely designed with a 10.5 volt LVD. and will also damage your batteries if overloaded or of you allow to drop voltage that low. Good luck with it.

[sensij]

All I have in the menu of the MPPT controller is a setting that says 'System Voltage' and that can be set to either 12v or 24v. But in the specification section it says maximum PV input for controller 275W at 12v / 550W at 24v. But it also says 12v/24v auto detect.

The error in your thinking, if you haven't figured it out yet, is that although the input amps will still be the same if you put more panels in series, the output amps from the CC at 12 V will be doubled. It is the output amp specification (written or implied by the power rating) that you would be violating by trying to put 400 W on it in 12 V mode. If you shop for them, you'll find that most CC's are rated and sold by output amp rating.

[Steevo25]

Now I can see what you mean. 400w / 24v is almost 17amps. When you try and charge 400w at 12v it becomes 34amps which is over the limits of the controller. I guess the controller will either sustain damage or it will throttle the output which makes the extra power completely pointless. Even if I use open circuit voltage in the equation it is still 12amps in and 24amps out at 12v.

Well I guess it is best left exactly as is. It is working as it is now so I guess don't try and fix it. Without spending a fair amount of money, I am not really going to get it any better than it is now.

It's either buy a new controller or move to 24volts throughout the system and then have to replace certain items like inverter etc.

[Mike90250]

if you have a real true MPPT controller (not a knock off with a sticker over the PWM label) it will downconvert the PV voltage to the battery voltage.
Batteries in parallel are not the best configuration, if you need more power, instead of 2, 12V batt in parallel, it's better to use 2, 6V golf cart batteries in series. The flow of electricity is perfectly equal in series circuit.
Most MPPT controllers have a max PV voltage, often about 50 or 75v for small ones, and 150V for the 40A and larger ones, but they all should be able to down-convert to battery voltage at +90% efficiency.